1. Field
The present disclosure generally relates to electro-absorption-modulator devices. More specifically, the present disclosure relates to a waveguide electro-absorption-modulator device.
2. Related Art
Silicon photonics is a promising new technology that can potentially provide low-power, high-bandwidth and low-latency interconnects in future computing systems. However, in order to implement silicon photonic links, efficient light modulators are needed. It is complicated to construct efficient light modulators because the electro-optic effect in silicon (Si) is weak. As a consequence, a number of alternate light-modulation mechanisms are being investigated. Two promising light-modulation mechanisms are the electro-absorption associated with the quantum-confined stark effect (QCSE) in SiGe/Ge quantum-well (QW) devices, and the electro-absorption associated with the Franz-Keldysh (FK) effect in tensile-strained germanium (Ge).
QCSE provides a strong electro-absorption mechanism, and has been used to make high-speed, low-power and compact opto-electronic devices using III-V materials. In practice, electro-absorption associated with the QCSE in a multiple QW structure that includes germanium QWs which are separated by silicon-germanium barriers can offer a much stronger electro-optic effect than a depletion-based silicon light modulator. Consequently, silicon-germanium QCSE devices can provide broadband operation with low driver voltage. In addition, the same QCSE device can be used as either a light modulator or a photo detector.
Similarly, increased electro-absorption (relative to silicon) can also be achieved using the FK effect in Ge1-xSix (for example, using the enhanced FK effect in tensile strained, epitaxial germanium-on-silicon). Because the FK effect takes place on a sub-picosecond time scale, the speed of the electro-absorption mechanism based on the FK effect is only limited by the RC delay, and can be designed to achieve very high bandwidth. Moreover, the same FK-effect device can also be used as a photo detector with high responsivity and high bandwidth.
Recently, a germanium FK light modulator has been demonstrated using an enhanced FK effect in tensile strained, epitaxial germanium-on-silicon using a vertical αSi—Ge-αSi structure. One problem associated with this approach is that the light modulator is fabricated in a different layer than the silicon waveguide layer. To address this, special vertical evanescent coupling from the crystal-silicon waveguide in a silicon-on-insulator (SOI) layer to a poly-silicon waveguide in the germanium layer can be used to implement an optical input/output (I/O) for the light modulator. However, the special vertical evanescent coupling increases the complexity of the optical device and increases optical losses.
Hence, what is needed is an electro-absorption-modulator device that does not suffer from the above-described problems.